The United States is lobbying European governments to back a British-led call to amend the EU arms embargo on Syria to put pressure on President Bashar al-Assad to enter into talks with the opposition.

John Kerry, the US secretary of state, has been urging the EU to reach consensus on a change that would allow weapons to be delivered to the rebels – though without any decision to do so at this stage.

Diplomatic sources said on Wednesday that Britain now has the support of France, Italy and Spain, while Germany is neutral. But Austria, Finland, Sweden and the Czech Republic are still opposed. Ambassadors of all 27 EU members have been called into the state department in Washington to be told of the latest US position ahead of a crucial foreign ministers' meeting in Brussels next Monday.

Speaking in Jordan on Wednesday, Kerry pledged publicly that the US and its EU allies would step up support for Syrian opposition forces to help them "fight for the freedom of their country" if Assad does not engage in talks with the rebels in good faith. Efforts are under way, with Russian backing, to convene a peace conference in Geneva some time in June.

In Britain, however, plans to amend the EU embargo are being complicated by disagreements between the Conservatives and Liberal Democrats and a row in Whitehall about the risks of supplying weapons to rebels fighting Assad's regime.

William Hague, the foreign secretary, who is with Kerry in Amman to discuss Syria, made clear the UK wants to alter the embargo to put pressure on Assad, but without yet deciding to send any weapons. Options include an amendment to allow weapons to be supplied to the opposition Syrian National Coalition or removal of the word "non-lethal" from the text. Another possibility is a short rollover of the embargo, which expires on 1 June, to see if the Geneva talks have any prospect of success – or deadlock. If there is no agreement the ban will lapse. That leaves open the possibility of unilateral decisions to supply arms, though in the UK that could clearly trigger a coalition crisis.

Nick Clegg, the Liberal Democrat leader and deputy prime minister, faces strong differences inside his own party. "There is a fallacy in the government position," Menzies Campbell, the senior Lib Dem foreign affairs expert, told the Guardian. "It is said that the purpose of giving more sophisticated weapons to the rebels is to send a message to Assad but his regime is so heavily supported by the Russians that if there was any imbalance Moscow would be bound to redress it."

Douglas Alexander, Labour's shadow foreign secretary, said that David Cameron had allowed speculation to build about the government's willingness to veto the EU embargo. "But how would the government prevent British-supplied weapons falling into the wrong hands?" he asked. "How does supplying weapons help to secure a lasting peace?"

The rebels and their supporters say the embargo must be lifted to help the anti-Assad camp resist overwhemingly superior Syrian government forces, which are equipped with tanks, aircraft and missiles and are supplied by Russia and Iran.

Labour says that regardless of the status of the embargo, any weapons deliveries would breach other EU and UN agreements that are binding on the UK.

Whitehall sources say the national security council, which is chaired by the prime minister, has "grave concerns" about the risk that weapons could fall into "the wrong hands", amid concern about the growing strength and prominence of jihadi-type groups such as Jabhat al-Nusra, which is linked to al-Qaida.

Alistair Burt, the foreign office minister for the Middle East, told MPs on Tuesday night: "There are no guarantees, but over time we have established a series of links with moderate groups who would have no vested interest in allowing equipment that might be used against them to fall into the wrong hands." Hague said on Monday that the UK could supply arms "only in carefully controlled circumstances, and with very clear commitments from the opposition side". Some arrangements would "necessarily be confidential."

Fighters with the Free Syrian Army, the mainstream rebel group, are being vetted in Jordan, where UK special forces and MI6 officers are believed to be involved. The CIA has reportedly been involved in training and coordinating arms deliveries from Saudi Arabia, Turkey and Qatar.

Information about the vetting process is shrouded in secrecy, but Hague said in a written parliamentary answer last week: "We are in close contact with the leadership of the Syrian National Coalition and Supreme Military Command Council in order to identify training beneficiaries that meet our criteria for the Law of Armed Conflict training. To ensure that the recipients of the training are legitimate members of the opposition all beneficiaries are carefully screened before they are invited to attend the training."

The fragmentation of rebel groups, the lack of a centralised command structure, the kidnapping of UN peacekeepers and human rights abuses are all sources of concern. The recent incident in which a rebel commander in Homs was filmed eating the heart or lung of a dead government soldier caused widespread revulsion.

The Syrian National Coalition released a video yesterday entitled "Fighter not a Killer" — a YouTube and TV advert about the norms of international humanitarian law and human rights law. "In light of the recent events that have occurred within the Free Syrian Army, we felt that it is imperative to outline and educate what is acceptable and what is not," said a spokesman, Khaled Saleh.

Oxfam also issued a warning against lifting the embargo: "Sending arms to the Syrian opposition won't create a level playing field," it said in a statement. "Instead, it risks further fuelling an arms free-for-all where the victims are the civilians of Syria. Our experience from other conflict zones tells us that this crisis will only drag on for far longer if more and more arms are poured into the country."An estimated 80,000 people have been killed in Syria since the uprising began in March 2011. Millions have fled their homes inside Syria or become refugees abroad.